The tuning of semiconductor band gaps can often provide significant performance increases and new applications for electronic, optoelectronic, and photocatalytic devices. Here, we study the band gaps of pure and nickeldoped zinc oxide thin films synthesized using the low-cost spray pyrolysis deposition method. Nickel concentration is varied from 0 to 15%, and the effects that this doping has on the electronic structure are analyzed. Using optical and synchrotron X-ray techniques, two regimes of band gap reduction via Ni doping are uncovered. For doping up to 4% Ni, there is a strong reduction in the gap, while continued doping up to 15% further reduces the gap, but to a lesser extent. The results are explained using X-ray spectroscopy and an Anderson impurity model. These tools show that the low doping case is driven by the interaction of the Ni 3d and O 2p states in both the valence and conduction bands. At high doping, the removal of Zn 3d states from the valence band and the change in Ni coordination from T d to O h both contribute to counteract the gap reduction. These results show how Ni can be used to tune the ZnO band gap over a large range useful for many applications.